1. Field of the Invention
The present invention relates to a differential detection read sensor for perpendicular magnetic recording suitable for high-density magnetic recording, a thin film head for perpendicular recording using the same and a magnetic recording apparatus using the head.
2. Description of the Related Art
As personal computers and workstations have been widespread rapidly, magnetic disk units as magnetic recording apparatuses forming the core of a nonvolatile file system have been required to increase the capacity quickly than ever. Increase of the capacity of the magnetic disk unit basically enhances a recording bit density, i.e., areal recording density.
The recording system in magnetic disk units currently commercially used is generally called an longitudinal recording method. This is a system in which a ferromagnetic film with high coercive force in the direction in parallel with a disk substrate surface is used as a recording medium, and then, the recording medium is magnetized in the substrate longitudinal direction so as to record information. In this case, a magnetization reversal part in which in-plane magnetizations are opposite to each other at a 180 angle corresponds to bit 1. To increase the areal recording density, it is necessary to increase the bit density in the disk circumferential direction (linear recording density) and the bit density in the disk radius direction (track density). The track density is currently limited by the forming process of geometrical track width and accuracy of head following of a recording/reproducing head. These are thus considered to be mainly the problems of processing and control system techniques. On the contrary, the linear recording density is thought to be limited in principle in that in light of the fact that the recording medium is an aggregate of ferromagnetic material crystalline particles, the linear recording density is associated with the magnetic stability of the aggregate. In the longitudinal recording system, magnetizations are opposite to each other around the magnetization reversal. A large inner magnetic field called a demagnetizing field in the direction to reduce the magnetization is generated around the magnetization reversal. The demagnetizing field forms, in the magnetization reversal part, a transition region with a finite width, that is, a region in which magnetization does not reach a sufficient value. When the bit intervals are narrowed and the adjacent magnetization transition regions are interfered with each other, there arises the disadvantage that the position of the magnetization reversal is shifted substantially. To increase the linear recording density, there is required a construction such that the medium is magnetized by overcoming the demagnetizing field. More specifically, the coercive force of the medium must be improved and the thickness of the recording magnetic film must be reduced to suppress the demagnetizing field. For this reason, the linear recording density is strongly limited by the construction and the magnetic property of the medium. In a standard longitudinal magnetic recording system, the ratio of the linear recording density to the track density is desirably about 10 to 15. When a recording density of 100 Gb/in2 is realized under the conditions, the bit interval in the circumferential direction is about 25 nm. When the necessary magnetic property of the medium in which the magnetization reversal width is below 25 nm is estimated by a simple model, the medium film thickness is below 15 nm and the coercive force is above 5 kOe.
When the coercive force exceeds 5 kOe, it is difficult to ensure the recording magnetic field enough to magnetize the medium. When the thickness of the Co alloy magnetic film is below 15 nm, the substantial volume of the medium crystalline particles is reduced. As compared with the magnetic anisotropic energy of the particles (i.e., the energy to stabilize the magnetization in the constant direction), the magnitude of the heat energy (i.e., the energy to disturb the magnetization) cannot be ignored. Thermal fluctuation of the magnetization is significant, so that there arises the problem of thermal signal loss in which the magnitude of the recording magnetization is reduced with time. To suppress the thermal signal loss, it is necessary to increase the coercive force or the volume of the crystalline particles. When the head magnetic field is limited as described above, the allowable coercive force has an upper limit. In addition, increase of the film thickness to increase the volume of crystalline particles means increase of the demagnetizing field. When attempting to ensure the crystalline particle volume of crystalline size in the longitudinal direction, the randomness of the magnetization distribution in the medium is large, resulting in increase of the medium noise. A sufficient signal S/N cannot be thus obtained. To realize the areal recording density exceeding 100 Gb/in2 while resisting the thermal signal loss and reducing the noise in the areal magnetic recording system, it is expected to be difficult in principle.
The perpendicular magnetic recording system is a system for forming the magnetization of a thin film medium so as to be perpendicular to the film surface, in which the recording principle is different from that of the prior art longitudinal magnetic recording medium. In other words, in the perpendicular magnetic recording system, since the adjacent magnetizations are not opposite to each other and are arranged in antiparallel, they are not affected by the demagnetizing filed. The magnetization transition region is expected to be very small to easily increase the linear recording density. The require to reduce the medium thickness is not as strong as that of the longitudinal recording. It is thus possible to ensure high resistance to the thermal signal loss. The perpendicular magnetic recording system is focused as a system essentially suitable for high-density magnetic recording. Various medium materials and constructions are proposed.
The perpendicular magnetic recording system has a system for employing a single-layer perpendicular magnetization film and a system for providing a soft magnetic underlayer adjacent to the disk substrate side of a perpendicular magnetization film. Using a two-layer perpendicular magnetic recording medium having a soft magnetic underlayer, there are considered the advantages: (1) a demagnetizing field generated on the surface of the recording layer can be reduced; and (2) the medium can be combined with a single pole type recording element to generate a large recording magnetic field having a steep distribution as compared with the ring head in the longitudinal recording. The technique is described in, for example, IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 657-662, Perpendicular Magnetic Recording—Evolution and Future. As the perpendicular magnetic recording medium of this system, there is studied a medium in which a perpendicular magnetization film made of a CoCr alloy is provided on a soft magnetic underlayer made of a soft magnetic film layer such as permalloy or Fe amorphous alloy.
Corresponding to a difference in the medium magnetization state between the longitudinal recording and the perpendicular recording, it is expected that the space distribution of a magnetic field applied from the medium to the reproducing sensor and the reproduced signal waveform of the perpendicular recording are different from those of the in-plane recording. Generally used as a reproducing sensor in the current longitudinal recording system is a so-called a shield type GMR (Giant Magnetoresistive) reproducing sensor. As shown in the upper part of FIG. 1, this is constructed such that one GMR reproducing element 12 is disposed between a pair of magnetic shields 11a and 11b made of soft magnetic materials. In the in-plane recording, a static magnetic field leaks from the reversal part of a medium magnetization 13. The GMR reproducing sensor senses the magnetic field and produces a Lorenzian waveform 17 as shown in the lower part of FIG. 1 as a reproducing signal. In this case, the pulse peak position corresponds to the reversal part.
The recording medium in the perpendicular recording has a recording layer 14 having perpendicular magnetic anisotropy and a soft magnetic underlayer 16 made of high-permeability ferromagnetic material, as shown in the upper part of FIG. 2. A medium magnetization 15 is arranged so as to be perpendicular to the medium surface. A static magnetic field is generated from a magnetization constant region between the reversal parts. The reproduced waveform from the GMR reproducing sensor at a low recording density is a step-like waveform 18, as shown in the lower part of FIG. 2. In this case, the zero-cross position of the step-like waveform corresponds to the reversal part.
A signal processing system for use in the current magnetic disk unit is assumed to be a one-peak type reproduced waveform as shown in the lower part of FIG. 1. In the system, decoding is impossible from the step-like reproduced waveform obtained from the system using the two-layer medium for perpendicular recording provided with a soft magnetic underlayer and the shield-type GMR reproducing sensor. To solve the problem, there are known the following three methods.
{circle around (1)} Differential process for reproduced signal
{circle around (2)} Change of a method for signal processing
{circle around (3)} Differential detection read sensor
Method {circle around (1)} passes a reproduced signal outputted from the head to a differential circuit before processing the signal. Method {circle around (2)} changes the method for signal processing so as to be suitable for the reproduced signal waveform. Both of them must largely change LSI of a currently used electric circuit system. The system noise and head noise are increased, resulting in requiring great improvement in S/N to the reproducing head. In method {circle around (3)}, the waveform obtained from the reproducing sensor has already been a one-peak type, so that any modification is not required to the system side. Method {circle around (3)} is the most realistic in view of constructing a small and inexpensive magnetic recording apparatus having large capacity.
As specific means for constructing a differential detection read sensor corresponding to perpendicular recording, IEEE Transactions on Magnetics, vol. 24, p2617 (1988) and Journal of Applied Physics, vol. 65, p402 (1989) disclose a reproducing sensor and system in which a circuit system is constructed so that two anisotropic magnetic resistance (AMR) elements indicate a reverse polarity response to a magnetic field so as to fetch differential voltage of each of both elements as a reproduced signal. Such a construction is called Gradiometer and can obtain the same effect when the two AMR elements are replaced by two GMR elements. FIG. 3 shows a schematic view of Gradiometer and shows a reproduced waveform obtained using the same. Two magnetic resistance elements (MR elements) 112a and 112b are disposed between the magnetic shields 11a and 11b. Both magnetic resistance elements 112a and 112b are constructed so that a voltage change to the magnetic field is reversed; that is, when the magnetic field in the same direction is applied, one of the magnetic resistance elements increases the voltage, and the other decreases the voltage. The sum of signals outputted from the respective elements is equivalent to the sensing of the differential of the magnetic field in the positions of the MR elements 112a and 112b. There is thus obtained a one-peak type reproduced waveform as shown in the lower part of FIG. 3 almost equal to the differential waveform of the signal of the lower part of FIG. 2. The specific construction of the reproducing sensor is shown in the upper part of FIG. 4. First, there are formed a magnetic shield 11a, a lower insulating gap 23a, an MR element 21a, and an electrode 22a for flowing to this a sensing electric current (an electric current for sensing a resistance change as a voltage change). Then, an intermediate insulating gap 24 is deposited (this corresponds to the interval between the above-mentioned two elements). A second MR element 21b and electrode 22b are formed to finally deposit an upper insulating gap 23b and a magnetic shield 11b. The construction thus formed disposes the two MR elements are disposed between the magnetic shields to be thoroughly independent electrically from each other. In the magnetic recording apparatus, the two elements are connected in series, and then, both ends are connected to exterior circuit systems 25 and 26 for reproducing operation, as shown in the lower part of FIG. 4. In the above-mentioned known art, it is reported that a thus-constructed reproducing system combined with the perpendicular recording medium obtains a one-peak type reproduced waveform.
To manufacture a head for use in the known art, MR film deposition and MR element patterning process are repeated twice. In this case, increase of the manufacturing cost due to the increased number of the processes becomes a problem. The patterning process to define the track width is repeated twice. When shifting between the two elements 21a and 21b is caused (this corresponds to shifting in the right and left direction in the upper part of FIG. 4), crosstalk to read the adjacent track signal may be caused to significantly deteriorate the S/N ratio of the reproduced signal. When two MR elements are made into the gap between the magnetic shields reduced with increase of the recording density, the thickness of the insulating films between the MR elements and between the MR element and the magnetic shield (the lower insulating gap 23a, the upper insulating gap 23b, and the intermediate insulating gap 24) must be reduced. It is very difficult to thoroughly provide electric insulation in the known art.
Japanese Published Unexamined Patent Application No. Hei 10-334422 discloses a technique for constructing a differential detection read sensor of another construction. As shown in the upper part of FIG. 5, in the head construction in this case, MR elements 21a and 21b are connected in parallel with a common electrode 27 and exterior circuit systems 25 and 26. When such a construction is employed, the patterning process of the MR element is required at least once. It is thus possible to avoid the problems of increase of the number of the processes indicated in the first prior art and of shifting between the elements. However, the problem of electric insulation in a lower gap layer 23a, an upper gap 23b and an intermediate insulating gap 24 between the two MR elements and the magnetic shields exists as in the first prior art.
In addition to the problem, both MR elements are connected in parallel to reduce the entire resistance change amount. Thus, only a very low reproducing sensitivity is expected, that is, there arises a new problem of significant reduction of the reproducing sensitivity. Specifically, when the resistance change amount of one MR element to the medium magnetic field is ΔR, in the first known art, a resistance change of 2×ΔR as the entire read sensor can be expected, while in the second prior art, it is ΔR/2. The two MR elements must be electrically connected in series as the requirement of the differential detection read sensor.